LASER INDUCED, FINE GRAINED, GAMMA PHASE SURFACE FOR NiCoCrAlY COATINGS PRIOR TO CERAMIC COAT

ABSTRACT

A process for forming a thermal barrier coating on a part comprising depositing an aluminum containing bond coat on the part, the bond coat comprising a surface; cleaning the surface to remove oxides and debris from the surface of the bond coat; forming a gamma phase layer proximate the surface of the bond coat; forming an aluminum oxide layer on the surface of the bond coat; and depositing a ceramic topcoat on the aluminum oxide layer on the bond coat.

BACKGROUND

The present disclosure is directed to a thermal barrier coating systemfor a component that is exposed to high temperatures, such as a gasturbine engine component (e.g. blades, vanes, etc.). More particularly,the present invention relates to the formation of a thermal barriercoating system and a method of forming a thermal barrier coating on ametal part that includes laser cleaning a surface of the metal part toremove undesirable oxides and residues from the surface of the part.

A gas turbine engine component, such as a blade tip, blade trailingedge, blade platform, blade airfoil, vane airfoil, vane trailing edge,or vane platform, is typically exposed to a high temperature and highstress environment. The high temperature environment may be especiallyproblematic with a superalloy component. Namely, the high temperaturesmay cause the superalloy to oxidize, or weaken which then decreases thelife of the component. In order to extend the life of the component, athermal barrier coating system (TBC system) may be applied to the entiresuperalloy component or selective surfaces, such as surfaces of thesuperalloy component that are exposed to the high temperatures and otherharsh operating conditions. A TBC system protects the underlyingmaterial (also generally called the “substrate”) and helps inhibitoxidation, corrosion, erosion, and other environmental damage to thesubstrate. Desirable properties of a TBC system include low thermalconductivity and strong adherence to the underlying substrate.

The TBC system includes a metallic bondcoat or oxidation resistantcoating and a ceramic topcoat (i.e., a thermal barrier coating or TBCtopcoat). The bondcoat is applied to the substrate and aids the growthof a thermally grown oxide (TGO) layer, which is typically alphaaluminum oxide, (Al₂O₃ or “alumina”). Specifically, prior to or duringdeposition of the TBC topcoat on the bondcoat, the exposed surface ofthe bondcoat can be oxidized to form the alumina TGO layer or scale. TheTGO forms a strong bond to both the topcoat and the bondcoat, and as aresult, the TGO layer helps the TBC topcoat adhere to the bondcoat. Thebond between the TGO and the topcoat is typically stronger than the bondthat would form directly between the TBC topcoat and the bondcoat. TheTGO also acts as an oxidation resistant layer, or an “oxidationbarrier”, to help protect the underlying substrate from damage due tooxidation.

In order for the TBC layer to adhere to the metallic bond coat (BC)(NiCoCrAlY for example), an oxide intermediate layer, a thermally grownoxide (TGO) is needed. Bond coat alloy in equilibrium consists ofAluminum (Al) poor phase (“gamma” phase, face centered cubic (fcc)crystal structure) and Al rich phase (“beta” phase, body centered cubic(bcc) crystal structure).

It should be noted that self-diffusion coefficient in bcc “beta” phaseis by order of magnitude higher than the one in fcc “gamma” phase due toits lower packing factor and lower coordination number.

Although the TGO is needed to bond the TBC to the metallic coat, the TGOcontinues to grow as the engine runs. The slight mismatch in volumebetween the TGO and TBC causes a build-up of stress between them thateventually causes the coating to fail. Therefore, it is important tohave the thinnest TGO possible at the start of the process, and to slowthe growth of the TGO during engine run conditions.

SUMMARY

In accordance with the present disclosure, there is provided a processfor forming a thermal barrier coating on a part comprising depositing analuminum containing bond coat on the part, the bond coat comprising asurface; cleaning the surface to remove oxides and debris from thesurface of the bond coat; forming a gamma phase layer proximate thesurface of the bond coat; forming an aluminum oxide layer on the surfaceof the bond coat; and depositing a ceramic topcoat on the aluminum oxidelayer on the bond coat.

In another and alternative embodiment, the cleaning comprises exposingthe surface to an energy beam focused on the surface of the bond coatand forming a liquid from the bond coat proximate the surface.

In another and alternative embodiment, the process further comprisesrapidly cooling the liquid into the gamma phase layer, and forming analpha aluminum oxide proximate the surface.

In another and alternative embodiment, the energy beam produces highintensity, short duration energy beam pulses.

In another and alternative embodiment, the process further comprisesinhibiting a non-alpha aluminum oxide layer from growing responsive tothe gamma phase layer proximate the surface.

In another and alternative embodiment, the process further comprisesdistributing the aluminum in the bond coat uniformly, so that the alphaaluminum oxide layer is even and has reduced internal stresses.

In another and alternative embodiment, the gamma phase layer proximatethe surface comprises a supersaturated aluminum content and low aluminumdiffusivity properties.

In another and alternative embodiment, the process further comprisesgrowing an initial alpha-alumina scale during the cleaning step.

In another and alternative embodiment, the gamma phase layer comprises athickness of about 1.5 microns.

In another and alternative embodiment, the gamma phase layer proximatethe surface of the bond coat is an aluminum diffusion inhibitor.

In another and alternative embodiment, the gamma phase layer impedesfast aluminum oxidation at the surface.

In another and alternative embodiment, the gamma phase layer allows fora very thin initial alpha-alumina scale to grow during the cleaningstep.

Other details of the process are set forth in the following detaileddescription and the accompanying drawings wherein like referencenumerals depict like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a turbine blade.

FIG. 2 is a cross-sectional view of the turbine blade of FIG. 1 where asection has been taken at line 2-2 (shown in FIG. 1) and show a thermalbarrier coating system overlying the airfoil of the turbine blade.

FIG. 3 is an exemplary cleaning process.

FIG. 4 is a phase diagram of an exemplary bond coat as a function oftemperature.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of turbine blade 10 of a gas turbineengine. Turbine blade 10 includes platform 12 and airfoil 14. Airfoil 14of turbine blade 10 may be formed of a nickel based, cobalt based, ironbased superalloy, or mixtures thereof or a titanium alloy. Turbine blade10 is exposed to high temperatures and high pressures during operationof the gas turbine engine. In order to extend the life of turbine blade10 and protect it from high stress operating conditions and thepotential for oxidation and corrosion, a thermal barrier coating (TBC)(shown in FIG. 2) is applied over airfoil 14 and platform 12 of turbineblade 10.

The exact placement of the TBC system depends on many factors, includingthe type of turbine blade 10 employed and the areas of turbine blade 10exposed to the most stressful conditions. For example, in alternateembodiments, a TBC may be applied over a part of the outer surface ofairfoil 14 rather than over the entire surface of airfoil 14. Airfoil 14may include cooling holes leading from internal cooling passages to theouter surface of airfoil 14, and the system 16 may also be applied tothe surface of the cooling holes.

FIG. 2 is a sectional view of turbine blade 10, where a section is takenfrom line 2-2 in FIG. 1. TBC system 16 is applied to an exterior surfaceof airfoil 14 and platform 12.

TBC system 16 may include bondcoat 18 and ceramic layer 20. Bond coat 18overlays and bonds to airfoil 14 and platform 12 while ceramic layer 20overlays and bonds to the thermally grown oxide (TGO) on the bond coat18. In the embodiment shown in FIG. 2, bond coat 18 may be applied toairfoil 14 and platform 12 at a thickness ranging from about 0.5 mils(0.0127 mm) to about 10 mils (0.254 mm). Ceramic layer 20 may be anythermal barrier coating (or “topcoat”) that is suitable for use onalumina forming bond coats and/or alloys. Non-limiting examples includezirconia stabilized with yttria (Y₂O₃), gadolinia (Gd₃O₃), ceria (CeO₂),scandia (Sc₂O₃), and other oxides known in the art. Ceramic layer 20 maybe applied by electron beam physical vapor deposition (EBPVD) or byplasma spray. Ceramic layer 20 may be deposited in thickness sufficientenough to provide the required thermal protection for bondcoat 18 andsubstrate 10.

Bond coat 18 may be a MCrAlY coating where M may be Ni, Co, Fe, Pt,Ni-base alloy, Co-base alloy, Fe-base alloy or mixtures thereof. In anembodiment, M may include Hf or Si or mixtures thereof. In an exemplaryembodiment, the bond coat 18 can comprise NiCoCrAlY. Bond coat 18 may beapplied to airfoil 14 and platform 12 by any suitable techniqueincluding, but not limited to, thermal spray processes such as lowpressure plasma spray (LPPS) deposition and high velocity oxyfuel (HVOF)deposition, physical vapor deposition such as cathodic arc deposition orchemical vapor deposition, and the like. In an embodiment, bond coat 18may be an aluminide bondcoat formed by techniques such as packcementation, chemical vapor deposition, and others followed byappropriate diffusion heat treatments.

The process 100 can be understood by referring also to FIG. 3. At aninitial state 120, the bondcoat 18 includes a surface 32 covered withcontaminant materials 34, such as oils, oxides and the like. Within thebond coat 18, a combination of beta phase bcc structure 36 and gammaphase fcc structure 38 is shown distributed throughout the bond coat 18.The beta phase bcc structure 36 can act as an Al reservoir for the bondcoat 18. The gamma phase fcc structure 38 can act as an Al diffusionmedia.

At step 130 cleaning and structural refinement takes place at and nearthe surface 32 of the bond coat 18. The cleaning is done at hightemperature and at an atmospheric pressure. In an exemplary embodiment,the cleaning temperature can be above 1350 degrees C., the point atwhich the NiCoCrAlY liquefies. An energy beam 40 is utilized to removethe contaminants 34 while also melting the bond coat 18 proximate thesurface 32, forming a liquid 42 from the bond coat 18 material. Theliquid 42 subsequently rapidly cools down into a fine grained layer 44.The grain refinement is achieved by very rapid melting andre-solidification of the bond coat 18 material at the surface 32 undervery high intensity, short duration energy beam pulses. In an exemplaryembodiment, the energy beam 40 can utilize a 50-100 nano-second pulseduration at 5-50 J/cm{circumflex over ( )}2. The local surface 32heating above the predetermined temperature, such as 1350 degreesCentigrade, can create the fine grained predominantly gamma phase layer44 proximate the surface 32. Raising the temperature this way causes themetal right at the surface to liquefy very quickly, and re-solidify veryquickly providing the desired fine grain structure.

The energy beam 40 can include a laser. The laser can include an yttriumaluminum garnet laser, ultraviolet, eximer, or carbon dioxide, fiber, ordisc laser. The laser can have a power range up to about 1000 watts.

The surface environment during the cleaning portion 130, can includeair, inert gas, water, or combinations thereof.

In an exemplary embodiment, a laser based cleaning system for removingcontaminants from the surface 32 may include: a laser 40 capable ofremoving the oxide layer and producing an aluminum oxide layer on thesubstrate surface 32; an optical system capable of focusing the laser atthe oxide layer; and a scanning system capable of directing the focusedlaser beam over the surface 32 to remove contaminants to produce thegamma phase layer 44 on the substrate 32.

In an exemplary embodiment, the Al-rich gamma phase layer 44 can beabout 0.5 microns. In an exemplary embodiment the gamma phase layer 44can range from about 0.25 microns to about 0.75 microns. In anotherexemplary embodiment, the gamma phase layer 44 can be about 1.5 micronsthick. The thickness of the layer 44 can be controlled by the amount ofenergy beam applied power to the surface 32. In an exemplary embodiment,the energy beam 40 applied power can range from about 500 Watts to about1000 Watts. In an exemplary embodiment, the surface 32 can be heated toabout 1350 degrees C. The surface 32 can be heated to temperatures fromabout 1350 degrees C. possibly much hotter, but not so hot as to startboiling off the Aluminum, so 2470 degrees C. would be the upper limit toproduce the desired gamma phase layer 44. At the higher temperatures, ascan be seen at FIG. 4, the bond coat 18 is converted predominantly togamma phase.

At 140 it can be seen that after cleaning and structural refinement, thebond coat 18 includes the gamma phase layer 44 with a smooth surface 32.The elements proximate the surface 32 of the metallic bond coat 18 tendto be Ni and Al. The molten alloy 42 freezes first at the location whereit is in contact with the bulk of the solid bond coat 18 and freezeslast where the molten alloy 42 is in contact with air or process gases.Thus, the disclosed process promotes gamma-alumina formation proximatethe surface 32. Under ambient conditions, the gamma phase normally haslower Al content than the beta phase. However, because of the rapidmelting and refreezing caused by the energy beam 40, all of the Al isstill present, so the gamma phase layer 44 is Al rich, or supersaturatedwith Al. Because the gamma phase is supersaturated with Al (i.e. itwants to get rid of Al) the gamma phase layer 44 has ability to create adense initial alpha-alumina layer of TGO and avoid the non-alpha phasealuminum.

The layer 44 is Al oversaturated which can be beneficial for the nextstep 150 of forming the aluminum oxide layer (TGO) 46 on the surface 32.The extremely fine grain structure 44 distributes the aluminum in thebond coat 18 very uniformly, so that the thermally grown oxide (TGO) 46is even and has fewer internal stresses.

At 160, it can be seen that once the thermally grown oxide 46 is formed,the TGO 46 will be inhibited from growing rapidly, and will grow slowlyas compared to a surface that was not treated by the disclosed process.The gamma phase 44 proximate the surface is reduced in aluminum contentand has poor diffusivity properties for aluminum. The slower TGO 46growth will result in good adhesion between the TBC 20 and the metallicbond coat 18. This also provides the benefit of longer life adhesion ofthe TBC 20, because the TGO 46 thickness is smaller and the stressesbetween the TGO 46 and TBC 20 will grow more slowly. The uniform gammalike fcc phase has an order of magnitude lower self-diffusioncoefficient that the beta bcc phase material. The gamma phase 44 impedesfast Al oxidation at the surface 32, and allows for a very thin initialalpha-alumina scale to grow during the disclosed laser cleaning process.The rapid cooling and formation of the gamma phase layer inhibits anon-alpha aluminum oxide layer from growing responsive to said gammaphase layer proximate said surface. The non-alpha aluminum oxides caninclude aluminum oxide in other phases, including the cubic γ and ηphases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombicκ phase and the δ phase that can be tetragonal or orthorhombic. Thealuminum oversaturation ensures that aluminum oxides are preferentiallycreated proximate the surface 32. During subsequent TGO 46 growth in anEB-PVD chamber that initial alpha-alumina creates conditions forpreferential alpha-alumina growth, which is less prone to further oxygendiffusion that causes TGO growth during part service life. TBCspallation happens when the TGO thickness reaches a certain value. Thedisclosed process provides for the initial TGO thickness and TGO growthrate coefficient to be smaller. The TGO growth coefficient for lasercleaned sample is about 2 times smaller than that for a grit blastedone.

The disclosed process relies upon surface chemical/phase compositionmodifications of the bond coat. In an exemplary embodiment, furtherprotection can include adding other alloying elements to the bond coatsurface 32, such as Pt, to further impeded the TGO growth.

The technical benefits of utilizing the disclosed process includesoptimized surface chemistry for optimal TGO growth.

Another technical advantage of the disclosed process is the developmentof a smooth surface 32 without steps and ledges that mitigates geometricstress build-up.

Another technical advantage of the disclosed process is that surfacetreated material is given a diffusion barrier and controller foraluminum so that the TGO starts and continues to grow uniformly with asmooth, non-stepped geometry.

Another technical advantage of the disclosed process is that surfacetreating with a laser or other energy source (like e-beam) is apreferred approach.

Another technical advantage of the disclosed process can include addingsupplemental elements to the surface by pre-coating (vapor deposition,sputtering, etc.) followed by laser processing that can also provideoptimal surface material for TGO growth and subsequent diffusionbarrier/control.

There has been provided a process. While the process has been describedin the context of specific embodiments thereof, other unforeseenalternatives, modifications, and variations may become apparent to thoseskilled in the art having read the foregoing description. Accordingly,it is intended to embrace those alternatives, modifications, andvariations which fall within the broad scope of the appended claims.

What is claimed is:
 1. A process for forming a thermal barrier coating on a part comprising: depositing an aluminum containing bond coat on the part, said bond coat comprising a surface; cleaning said surface to remove oxides and debris from the surface of the bond coat; forming a gamma phase layer proximate the surface of the bond coat; forming an aluminum oxide layer on said surface of said bond coat; and depositing a ceramic topcoat on the aluminum oxide layer on the bond coat.
 2. The process according to claim 1, wherein said cleaning comprises exposing the surface to an energy beam focused on the surface of the bond coat and forming a liquid from the bond coat proximate the surface.
 3. The process according to claim 2, further comprising: rapidly cooling the liquid into said gamma phase layer; and forming an alpha aluminum oxide proximate said surface.
 4. The process according to claim 2, wherein said energy beam produces high intensity, short duration energy beam pulses.
 5. The process according to claim 1, further comprising: inhibiting a non-alpha aluminum oxide layer from growing responsive to said gamma phase layer proximate said surface.
 6. The process according to claim 1, further comprising: distributing the aluminum in the bond coat uniformly, so that the alpha aluminum oxide layer is even and has reduced internal stresses.
 7. The process according to claim 1, wherein said gamma phase layer proximate the surface comprises a supersaturated aluminum content and low aluminum diffusivity properties.
 8. The process according to claim 1, further comprising: growing an initial alpha-alumina scale during the cleaning step.
 9. The process according to claim 1, wherein the gamma phase layer comprises a thickness of about 1.5 microns.
 10. The process according to claim 1, wherein said gamma phase layer proximate the surface of the bond coat is an aluminum diffusion inhibitor.
 11. The process according to claim 8, wherein said gamma phase layer impedes fast aluminum oxidation at the surface.
 12. The process according to claim 8, wherein the gamma phase layer allows for a very thin initial alpha-alumina scale to grow during said cleaning step. 